Depends on who you are with.
If you are with clients then you need to act in a professional manner. Displaying stress or just general unhappiness in front of clients is not professional and should be avoided.
If you are with your team (especially the person directly above you/who you report to) you should have the freedom to express unhappiness and stress. Being open with to your team is part of a healthy work environment. This doesn't mean complaining about every little thing but just being honest with where you're at.
It should never be required that you act stressed or unhappy when it's not how you are feeling. If a company is expecting that of you then they are encouraging an unhealthy work environment. This can be either due to your company having a distorted view on productivity and a healthy work load or you boss might just be stressed and unhappy and feel like if he's stressed by his work you should be too. Either way it doesn't make for a good working environment. It might change but realistically you're probably better off looking for an employer with healthy expectations from their employees.
This is just what I have seen from personal experience and ultimately everybody will have a different opinion. Take it with a grain of salt and decide what's important in a work environment for you before making any changes. You should never quit your job without being aware of your situation and knowing how hard it's going to be to find a new job.